The present invention relates generally to detectors for locating hidden objects. More particularly, the present invention relates to detectors for locating objects hidden behind walls, ceilings and floors, for locating metallic and non-metallic buried objects, and further for locating cavities within solid objects.
I. Detection of objects hidden behind wood walls, ceiling and floors
A common problem faced by anyone attempting to hang a picture or cabinet is how to precisely locate between-wall studs, so a sturdy hook may be attached or clearance may be provided for the cabinet. Since wall studs are usually covered by sheetrock or wallboard and finished-off, stud location is not visible. A similar problem arises when hanging plants and lamps from the ceiling, or when nailing down squeaky floorboards and stair steps. General methods for locating studs and joists include tapping with a hammer, searching for nails with a magnetic compass, and random piercing with a nail. Hammer tapping and magnetic compass searching are unreliable and time-consuming, and random piercing is destructive. Once a nail is located, it may be off-center. Also, the stud may be warped, making it impossible to deduce accurate stud location at any distance from the nail.
These conventional methods were vastly improved when electronic wall stud sensors became commercially available. The user places the sensor flat against the wall and scans it laterally across the extent of the wall. When it passes over a stud, a vertical series of LED's indicate the presence of the stud behind the wall. The sensor is based on dielectric density sensing. U.S. Pat. No. 4,099,118 describes a portable electronic wall stud sensor having capacitor plates and circuitry for detecting changes in the capacitive charge due to changes in the dielectric constant in the wall adjacent the sensor. U.S. Pat. No. 4,464,622 describes a similar capacitive sensor with calibration means and means for detecting an AC line in the wall.
Dielectric density sensing has limitations. If a small air gap forms between the sensor and the wall, the device becomes inoperative due to the substantial change in density adjacent the two sensing plates that are internal to the unit. It is therefore difficult or impossible to locate studs on rough or highly textured surfaces. Another limitation is that stud detection is directly affected by the dielectric constant of the intervening wall material. Sheetrock, plywood, particle board, and dense hardwoods vary in dielectric constant to such an extent that a dielectric sensor generally only works on sheetrock and not on plywood walls, wood floors, stair steps, furniture or cabinetry. Furthermore, these conventional sensors cannot detect cavities behind walls or within objects.
Therefore, there is an unsatisfied need for a new detector capable of locating hidden metallic objects such as conduits, electrical wiring and nails, and non-metallic objects, such as pipes, studs and joists behind wood walls, ceilings and floors. This detector should also locate cavities behind the latter structures. It should not be sensitive to the condition of these structures (i.e., accumulated dirt, rough or highly textured surfaces), or to its distance from these structures (i.e., it does not need to be placed flat against these structures ). It should not be directly affected by the dielectric constant of the intervening wall, ceiling and floor materials, and should work on almost all structures, including without limitation, sheetrocks, plywood, particle board, dense hardwoods, such as wood floors, stair steps, furniture or cabinetry having different dielectric constants, and tiles.
This new detector should have a first surface cancellation effect, with a fixed and controllable detection or depth adjustment; it should be portable, light weight, easy to use, relatively inexpensive, and should have a low power emission which helps comply with the requirements of Part 15 of the Federal Communications Commission's Rules. Furthermore, this new detector should not interfere with the operation of other detectors and telecommunications and wireless equipment in the vicinity. It should also be adaptable for automating the construction process, such as for use with nail guns.
II. Detection of objects behind masonry and cement structures
Locating hidden metallic and non-metallic objects and cavities behind masonry and cement structures presents further complications. Conventional methods of locating embedded objects rely on trial and error methods, which include drilling several holes in the structures, in the general area where the objects are believed to be hidden. Oftentimes, this method causes damage to the objects and to the drill equipment. Conventional magnetic methods have limited applications, such as detecting copper wiring or aluminum conduit.
Therefore, there is a need for a detector which accurately detects objects and cavities behind masonry and cement structures. This detector should be reliable, portable, inexpensive and simple to use.
III. Detection of Underground Objects
Previously, metallic underground pipes were used almost exclusively in the transportation of natural gas. The location of the buried metallic pipes was relatively simple since metal reflects high frequency electromagnetic waves which can be easily detected. However, underground metal pipes have inherent problems. They are subject to corrosion to differing degrees, they are difficult to install, and they are becoming more difficult and expensive to purchase. As a result of these limitations, other types of pipes have become popular. Polymeric pipes, being virtually non-corrosive, light, easily installed and relatively inexpensive are rapidly replacing metallic pipes.
An ever increasing problem facing the natural gas distribution companies, municipal government agencies, other public utilities and contractors is the rapid and accurate location of buried polymeric pipe lines. Since underground plastic pipes cannot be located with conventional metal detectors, sub-surface detectors of non-metallic and metallic objects have evolved. Examples of these detectors are shown in the following patents:
______________________________________ Patent/Appliation No. Patentee Issue Date ______________________________________ U.S. Pat. No. 4,062,010 Young et al. December 6, 1977 U.S. Pat. No. 4,028,707 Young et al. June 7, 1977 U.S. Pat. No. 3,967,282 Young et al. June 29, 1976 U.S. Pat. No. 3,806,795 Morey et al. April 23, 1974 U.S. Pat. No. 4,905,008 Kawano et al. February 27, 1990 U.K. 2,238,201 Cordes November 17, 1989 ______________________________________
Many of these detectors operate by emitting a radar-like signal, reflecting it off of a target, receiving the reflected wave, and operating on it. The target reflects the waves differently than its environment because of its different dielectric constant. The surface of the ground minerals and other items have different dielectric constants and produce signals which may give deceptive information. Water content, in particular, varies the dielectric constant substantially and makes consistent detection of targets difficult at best.
Some of the above listed detectors have tried to compensate for the ground effect in different ways. One representative patent is Young et al., U.S. Pat. No. 4,062,010, which describes an underground pipe detector which addresses the problem of compensation for variations in the dielectric constant without resorting to a dual antenna system, and teaches the use of a single antenna having transmitting and receiving sections. An electrical impulse source transmits a radar-like signal through an antenna into the ground and is reflected by a target. The reflected signal or echo is detected by the antenna and an analog-to-digital converter converts it to a digital form which may be operated on, stored and recalled.
Compensation for different dielectric constants is accomplished by sampling the dielectric constant near the target area and comparing the resultant signal with that received from the target area. The compensation is handled electronically by storing a first signal in order that it can be recalled for comparison with a second signal received from the target. The patented detector seems limited to the processing of converted digital signals, since admittedly, the equipment necessary to operate on an analog signal in a similar manner would be of such magnitude as to be unusable in the field. An inherent problem in this system is that it is relatively large and expensive. It requires a trained operator to interpret the collected data.
U.S. Pat. No. 3,806,795 to Morey et al. relates to a geophysical survey system for determining the character of the subterrain by analysis of reflections from electromagnetic pulses radiated into the ground. The system repetitively radiates into the ground a short duration electromagnetic pulse having a rise time in the order of 1 nanosecond. The antenna which radiates the pulse into the ground is employed to receive the reflections of the pulse. The received signals are coupled through a transmit-receive network to a receiver which permits the input signal waveform to be reconstructed from a sequence of samples taken by the receiver. The system is capable of generating a profile chart indicating the magnitudes of the reflected signals and the depths at which the reflections occurred. However, this system is bulky and expensive.
U.S. Pat. No. 4,905,008 to Kawano et al. relates to a radar type underground searching apparatus for detecting the presence and location of buried objects such as underground gas pipes. This apparatus includes a pulse generating unit which periodically generates a pulse, and a transmitting antenna through which the pulse is sent into the ground. Pulses reflected from an object in the ground and reaching the ground surface are detected by a receiving antenna, and a reflected wave corresponding to each pulse wave received by the receiving antenna is amplified by a radio-frequency amplifier. The output of the radio frequency amplifier is sampled with a sampler such that each sample is delayed by a fixed period from the transmitting timing of each pulse wave. The presence of objects will be detected by the presence of peaks in the low-frequency signal caused by the reflection from the objects and the depths of the objects will be determined by the time at which the peaks appear on a screen. However, this system is also relatively large in size, expensive and complicated to use.
U.K. patent application No. 2,238,201 discloses a ground probing radar for locating objects buried in the ground such as pipes and cables. The radar uses a radio transmission which varies the frequency of a controlled oscillator by feeding it with random or pseudo-random voltages or currents. The oscillator drives an impulse generator. The transmission is received by a sampling gate triggered by a voltage controlled delay receiving the output from a monostable. The voltage controlled delay is set by a ramp voltage produced by a digital to analog converter driven by a counter receiving the output from the monostable. However, the disclosed radar is not too economical.
In yet another attempt to circumvent the problem of non-conductivity of plastic pipes, tracer wires are buried above these pipes. The tracer wires carry an electrical current, and can be located by metal detecting systems. However, tracer wires are expensive, and are eventually destroyed, broken or corroded and, when repairs are made to the pipe, broken wires are often not replaced or repaired. In many instances, tracer wires are never installed.
There is therefore a great and still unsatisfied need for a new locator for detecting objects buried underground, which is portable, easy to use, relatively inexpensive, and which has a low power emission that helps comply with the requirements of Part 15 of the Federal Communications Commission's Rules. Furthermore, this new detector should be readily usable in security applications such as for locating guns and similar objects in suitcases.